This invention relates to turbochargers that have rotor assemblies that rotate at high speed and are used on internal combustion engines where the turbine component of the rotor is driven by high temperature exhaust gas.
Turbochargers are produced currently in the millions per year for use on both gasoline and diesel engines. Small units used on passenger car diesel and gasoline engines have been designed to reduce the rotor inertia and minimize turbocharger rotor lag during engine acceleration.
Much effort was expended in the early years of turbocharger development to produce a bearing system that exhibited sufficient durability to make a small size turbocharger commercially viable. Bearing systems for small turbochargers run at very high speeds, some exceeding 200,000 rpm, must be capable of mass production manufacturing methods, be low in cost, and easily serviced in the field.
Research and development during the 1960""s resulted in the perfection of lubricated floating sleeve-bearing systems that were capable of suppressing the problems of shaft instability, had acceptable friction losses and achieved satisfactory durability when used on a variety of internal combustion engine turbochargers. Several of these successful bearing systems are illustrated in U.S. Pat. Nos. 3,056,634; 3,096,126; 3,390,926; 3,993,370; and 4,641,977. The bearings of the patents listed above generally solved the stability problem by using a free-floating bushing between the rotating shaft and its stationary supporting member with films of lubrication between its inner surface and the rotating shaft and also between its outer surface and the stationary supporting member. In these systems, the free-floating bushings were free to rotate, but at speeds only a fraction of the speed of the rotating shaft and were free to move radially in order to allow the rotating assembly to find and rotate about its center of mass, and the inner and outer oil films provided the necessary lubrication to prevent wear and provided a cushion against vibration and shock loads.
In the sleeve bearing systems described above, it was necessary to provide a thrust bearing to sustain the axial loads imposed on the rotating assembly by the actions of the compressor and turbine wheels used in the turbochargers, and a collar was provided on the rotating shaft to bear against a stationary thrust member. However, the high rotational speed of the collar attached to the shaft resulted in a high thrust frictional loss which, in addition to the frictional losses of the sleeve bearings, resulted in a substantial total frictional loss for the complete bearing system. Such high frictional losses substantially reduce the mechanical efficiency of turbochargers, and it has long been desirable to use anti-friction bearings.
U.S. Pat. No. 4,370,106 discloses a lubricated bearing system for a turbocharger rotor consisting of an anti-friction ball bearing at its compressor end and a sleeve bearing at its turbine end. In this system, both the anti-friction bearing and the sleeve bearing are mounted in a non-rotating elongated cylinder. The cylinder containing the ball bearing and sleeve bearing is prevented from rotating by a square portion at the compressor end that engages stops in the stationary housing member. Lubricant is provided between the non-rotating cylinder and the supporting housing to provide damping for eccentric motion of the rotor due to residual imbalance. In this bearing system, however, the differential speed between the sleeve bearing and rotor is the very high rotative speed of the rotor. Since sleeve bearing frictional losses are proportional to the square of the differential rotating speed, this system has an inherent higher frictional loss than a full-floating sleeve bearing system. Also, since the non-rotating cylinder that contains the bearings must engage the stationary housing member, it carries the full thrust load of the rotor. The residual imbalance in the rotor forces the non-rotating cylinder to move orbitally, causing the mating surfaces to be subject to fretting. Thus a solid film lubricant must be placed between the mating surfaces to mitigate the fretting problem; however, this problem remains an inherent disadvantage with this type of non-rotating cylinder system and contributes to a limited service life in the field.
The fretting problem inherent with non-rotating systems that are allowed to move radially is solved in the lubricated bearing system disclosed in U.S. Pat. No. 4,641,977. In this bearing system, a ball bearing is mounted in an elongated cylinder that has a radially extending flange at one end. The elongated cylinder is lubricated and free to move radially to a limited degree and free to rotate in the stationary supporting member. The radially extending flange engages the stationary housing to carry the thrust load of the rotor. However, since the elongated cylinder rotates at relatively low speeds, the thrust losses are minimal. In this bearing system, a lubricated free-floating sleeve bearing is located at the opposite end of the elongated cylinder to complete the bearing system of carrying the rotor. The frictional losses with this system are reduced due to the ball bearing and floating sleeve bearing; thus, the mechanical efficiency of the system is relatively high compared to prior bearing systems.
My pending U.S. patent application Ser. No. 09/978,935, discloses a lubricated bearing system employing two angular contact ball bearings mounted in a rotating sleeve that achieves both low friction losses and excellent rotor stability.
Thus, commercial turbochargers have, for years, used engine lubricating oil fed to the turbocharger bearings to achieve rotor stability and satisfactory durability. This necessitates the design of sealing devices between the turbocharger rotating shaft and the bearing housing to prevent oil leakage into the compressor chamber, and into the turbine chamber where the oil can become carbonized due to the high temperature environment in the turbine component of the machine.
Oil leakage in turbochargers has been a persistent problem and a completely satisfactory solution has yet to be found. The small ring seals now used in commercial turbochargers to confine the bearing lubrication must allow some running clearance to eliminate friction and wear; thus, they cannot be totally leak free during some engine operating conditions. For example, when the engine is running at low idle speeds and when a vacuum exists in the air intake system due to the pressure drop across the air cleaner, there is a tendency for oil leakage to occur into the compressor chamber which can subsequently be carried into the engine air intake manifold. Thus, it would be desirable to eliminate the use of lubricating oil in turbochargers in order to totally eliminate the problem of oil leakage in turbochargers.
In addition, electric motor-assisted turbochargers are well known and have been proven to help overcome the turbo lag problem, improve engine performance, and reduce smoke and emissions during the engine acceleration period. The electric generating capability of such electric motor-assisted turbochargers has the potential for eliminating the waste gates used on commercial turbocharged engines and can be utilized to feed electric current back into the vehicle""s electric system. However, prior motor-assisted turbocharger systems have suffered from certain deficiencies and complications.
This invention provides for the use of anti-friction ball bearings in a unique arrangement, does not require the use of lubricating oil from the internal combustion engine lubricating system and permits close coupling of the bearings and a compact turbocharger.
In this invention, a turbocharger bearing housing forms a coolant water jacket with an inner bearing engaging portion that has two bearing engagement surfaces engaged with the outer races of two anti-friction ball bearings whose inner races carry the rotating shaft, turbine and compressor of the turbocharger. The anti-friction ball bearings are, preferably, angular contact ball bearings, and the two bearing engagement surfaces of the bearing housing are closely spaced, for example, a length of about the axial length of the compressor wheel or less, providing a turbocharger shaft of minimal length and substantially reducing the thermal expansion of the shaft. In the invention the coolant water jacket protects the anti-friction ball bearings from exposure to the extreme heat of the exhaust gas driven turbine, notwithstanding their increased proximity due to the shortened turbocharger shaft, and the bearing housing may be thinned, or otherwise adapted at the two bearing engagement surfaces for increased protection of the anti-friction ball bearings by the coolant water jacket.
The invention also overcomes the problems and complications of motor-assisted turbocharger systems by providing an economical combination of a motor-generator with a compact turbocharger that combines all the essential elements of a motor-assisted turbocharger in a single compact device. In the invention an external motor-generator is carried by the compact turbocharger and its motor is connected to the turbocharger rotor assembly by a permanent, solid connector and stays connected throughout the entire operating range of the turbocharger. The electronic motor-generator control is mounted on the motor housing and energizes the motor from battery power during the engine acceleration period up to approximately the torque peak speed; thereafter, the control changes to a generator mode when excess energy is available in the engine exhaust gas. Mounting the electronic control on the motor housing through which engine intake air is ducted allows a very short connection between motor and controller, and effective cooling of the control elements and the motor windings.